Thomas André

A model-based motion compensated video coder with JPEG2000 compatibility

We present a highly scalable wavelet-based video coder, featuring a scan-based motion-compensated temporal wavelet transform (WT) with lifting schemes which have been specially designed for video. Output bitstream is compatible with JPEG2000, as it is used to compress temporal subbands (SBs). Rate allocation among SBs is done by means of an optimal algorithm, which requires SBs rate-distortion (RD) curves. We propose a model-based approach allowing us to compute these curves with a considerable reduction in complexity. The use of temporal WT and JPEG2000 guarantees high scalability.

JPEG2000-compatible scalable scheme for wavelet-based video coding

We present a simple yet efficient scalable scheme for wavelet-based video coders, able to provide on-demand spatial, temporal, and SNR scalability, and fully compatible with the still-image coding standard JPEG2000. Whereas hybrid video coders must undergo significant changes in order to support scalability, our coder only requires a specific wavelet filter for temporal analysis, as well as an adapted bit allocation procedure based on models of rate-distortion curves. Our study shows that scalably encoded sequences have the same or almost the same quality than nonscalably encoded ones, without a significant increase in complexity. A full compatibility with Motion JPEG2000, which tends to be a serious candidate for the compression of high-definition video sequences, is ensured.

Entropy-Based Distortion Measure and Bit Allocation for Wavelet Image Compression

Quality criteria for image coding are often based on mean square error. However, this is not always a relevant measure of visual quality at low bit rates. Here, we investigate the properties of a distortion measure based on the conditional differential entropy of the input signal given its quantized value. The proposed measure appears to be a correct representation of the amount of information lost by quantization. An adaptive bit allocation algorithm is proposed in order to take advantage of this criterion. Experimental results illustrate the behavior of the proposed distortion measure and exhibit interesting visual properties for low bit-rate subband image coding.

(N,0) motion-compensated lifting-based wavelet transform

Motion compensation has been widely used in both DCT- and wavelet-based video coders for years. The recent success of the temporal wavelet transform based on motion-compensated lifting suggests that a high-performance, scalable wavelet video coder may soon outperform the best DCT-based coders. However, motion-compensated lifting does not implement exactly its transversal equivalent unless certain conditions on motion are satisfied. We review those conditions, and we discuss their importance. We derive a new class of temporal transforms, the so-called 1-N transversal or (N,0) lifting transforms, that are particularly interesting if those conditions on motion are not satisfied. We compare experimentally the 1-3 and 5-3 motion compensated wavelet transforms for the ubiquitous block-motion model used in all video compression standards. For this model, the 1-3 transform outperforms the 5-3 transform due to the need to transmit additional motion information in the latter case. This interesting result, however, does not extend to motion models satisfying the transversal/lifting equivalence conditions.

Optimal Motion Estimation for Wavelet Motion Compensated Video Coding

Wavelet-based coding is emerging as a promising framework for efficient and scalable compression of video. Nevertheless, a number of basic tools currently employed in this field have been conceived for hybrid block-based transform coding. This is the case of motion estimation, which generally aims to minimize the energy or the absolute sum of prediction error. However, as wavelet video coders do not employ predictive coding, this is no longer an optimal approach. In this paper we study the problem of the theoretical optimal criterion for wavelet-based video coders, using coding gain as merit figure. A simple solution has been found for a peculiar but useful class of temporal filters. Experiments confirm that the optimally estimated vectors increase the coding gain as well as the performance of a complete video coder, but at the cost of an augmented complexity.

Compression artifacts reduction using variational methods : Algorithms and experimental study

Many compression algorithms consist of quantizing the coefficients of an image in a linear basis. This introduces compression noise that often look like ringing. Recently some authors proposed variational methods to reduce those artifacts. They consists of minimizing a regularizing functional in the set of antecedents of the compressed image. In this paper we propose a fast algorithm to solve that problem. Our experiments lead us to the conclusion that these algorithms effectively reduce oscillations but also reduce contrasts locally. To handle that problem, we propose a fast contrast enhancement procedure. Experiments on a large dataset suggest that this procedure effectively improves the image quality at low bitrates.

A smoothly scalable and fully JPEG2000-compatible video coder

In this paper, we analyze the scalability properties of the JPEG2000-compatible video encoder presented in M. Cagnazzo et al., (2004), and we improve its performances by presenting a new technique for an efficient motion vectors (MVs) encoding, producing a motion bitstream also compatible with JPEG2000. Our study shows that, thanks to our encoding strategy and to our peculiar temporal filters, scalably encoded sequences have the same or almost the same quality than non-scalably encoded ones: this is what we call smooth scalability. We also compared our encoder performances with the recent H.264 standard, showing comparable or sometimes better performances.

Entropy-Based Distortion Measure for Image Coding

Classical quality criteria for image coding are based on the mean square error. We investigate here the properties of a distortion measure based on differential entropy of the error signal. The proposed measure leads to an interesting alternative code design criterion. An adapted bit allocation algorithm is proposed in order to take advantage of this criterion. Experimental results illustrate the behavior of the proposed distortion measure and exhibit interesting psycho-visual properties.

Modeling the Motion Coding Error for Mcwt Video Coders

In motion-compensated wavelet based video coders, a very precise motion estimation is necessary. However, a motion vectors field of high precision is expensive in binary resources and requires a great place in the bitstream compared to the wavelet coefficients. Thus, we need to reduce the cost of the motion information. To this end, we propose an approach based on a scalable lossy coding of high-precision motion vectors. It allows to optimize the trade-off between motion bit-rate and wavelet coefficients bit-rate, strongly reduces the motion cost, and thus, increases the coder performances at low bit-rate. Obviously, this lossy motion coding has an impact on the decoded sequence. In this paper, we evaluate this impact by establishing a theoretical distortion model for the motion coding error. This model allows the realization of an optimal model-based bit-rate allocation between wavelet subbands and motion vectors. The experimental validation of the model gives satisfactory results